Systems and methods for separation of liquid and gaseous fuel for injection

ABSTRACT

A method for an engine may comprise, on board a vehicle, supplying fuel from a fuel tank to a fuel separator, wherein the fuel comprises a gaseous fuel solubilized in a liquid fuel, desolubilizing the gaseous fuel from the liquid fuel in the fuel separator, and separating the gaseous fuel from the liquid fuel in the fuel separator.

BACKGROUND AND SUMMARY

Compressed natural gas (CNG) is a high octane fuel that is beneficialfor reducing engine knock, for reducing hydrocarbon emissions in coldstart events, and for reducing carbon dioxide emissions during engineoperations. However, CNG has a low energy density compared to liquidhydrocarbon fuels, such as diesel fuel or gasoline. To increase therange and total fuel quantity stored in a vehicle, CNG may be utilizedin conjunction with gasoline or diesel fuel, requiring the vehicle toswitch between fuels for optimal performance. However, inclusion ofseparate fuel tanks, one tank for gaseous fuel and one tank for liquidfuel, may not be suitable in a vehicle due to space constraints. Apreferable system may be one that stores liquid fuel and pressurizedgaseous fuel together in the same fuel tank. In particular, CNG issubstantially soluble in gasoline or diesel fuel when stored together ata relatively low pressure (˜100 psi).

The inventors herein have recognized potential issues with the aboveapproach. Namely, fuel metering accuracy may be decreased because liquidfuel supplied from the fuel tank may contain a mixture of liquid fueland solubilized gaseous fuel, and the gaseous fuel may form bubblesduring fuel injection. Furthermore, formation of gaseous fuel bubblesmay adversely change the dispersion of injected fuel in the engine,reducing fuel economy and engine efficiency.

One approach which at least partially addresses the above issuescomprises a method for an engine, comprising on board a vehicle,supplying fuel from a fuel tank to a fuel separator, wherein the fuelcomprises a gaseous fuel solubilized in a liquid fuel, desolubilizingthe gaseous fuel from the liquid fuel in the fuel separator, andseparating the gaseous fuel from the liquid fuel in the fuel separator.

In another embodiment, a method may comprise on board a vehicle, storingfuel in a fuel tank, wherein the fuel comprises a gaseous fuelsolubilized in a liquid fuel, during a first condition, desolubilizingand separating the gaseous fuel from the liquid fuel in a fuelseparator, and supplying the gaseous fuel and the liquid fuel to fuelinjectors, and during a second condition, supplying fuel from the fueltank to the fuel injectors while bypassing the fuel separator.

In another embodiment, a fuel system may comprise a fuel tank on board avehicle, the fuel tank including a gaseous fuel dissolved in a liquidfuel, a fuel injection system downstream from the fuel tank, the fuelinjection system including gaseous fuel injectors and liquid fuelinjectors, and a fuel separator fluidly coupled between the fuel tankand the fuel injection system.

In this way, the technical result may be achieved in that a consistentdispersion of injected fuel in engine cylinders can be maintained whileproviding more robust and accurate fuel injection control so that engineemissions may be reduced and fuel economy and engine efficiency may beincreased. The above advantages and other advantages, and features ofthe present description will be readily apparent from the followingDetailed Description when taken alone or in connection with theaccompanying drawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 schematically depicts an example cylinder of an internalcombustion engine.

FIG. 2 shows a schematic depiction of the engine of FIG. 1 and a fuelsystem configured to operate on a mix of gaseous fuel and liquid fuel.

FIG. 3 shows a flow chart of an example method for operating the engineand fuel system of FIGS. 1-2.

FIG. 4 shows an example timeline for operating the engine and fuelsystem of FIGS. 1-2.

DETAILED DESCRIPTION

The present description relates to systems and methods for supplyinggaseous fuel and liquid fuel to an engine and fuel system. An exampleinternal combustion engine and fuel system is illustrated in FIGS. 1 and2. FIG. 3 illustrates a flow chart for a method of operating an engineand fuel system to supply gaseous fuel and/or liquid fuel to the engine.FIG. 4 is an example timeline representing supplying gaseous fuel and/orliquid fuel from the fuel system to the engine during various engineoperating conditions.

Turning now to FIG. 1, it depicts an example embodiment of a combustionchamber or cylinder of internal combustion engine 10. Engine 10 may becontrolled at least partially by a control system 13, includingcontroller 12, and by input from a vehicle operator 130 via an inputdevice 132. In one example, input device 132 includes an acceleratorpedal and a pedal position sensor 134 for generating a proportionalpedal position signal PP. Cylinder (e.g., combustion chamber) 14 ofengine 10 may include combustion chamber walls 136 with piston 138positioned therein. Piston 138 may be coupled to crankshaft 140 so thatreciprocating motion of the piston is translated into rotational motionof the crankshaft. Crankshaft 140 may be coupled to at least one drivewheel of the passenger vehicle via a transmission system. Further, astarter motor may be coupled to crankshaft 140 via a flywheel to enablea starting operation of engine 10.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 1 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 162 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 162 may be disposed downstreamof compressor 174 as shown in FIG. 1, or may alternatively be providedupstream of compressor 174.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be any suitable sensor for providing an indication ofexhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO(universal or wide-range exhaust gas oxygen), a two-state oxygen sensoror EGO (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor.Emission control device 178 may be a three way catalyst (TWC), NOx trap,various other emission control devices, or combinations thereof.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 via actuator 152.Similarly, exhaust valve 156 may be controlled by controller 12 viaactuator 154. During some conditions, controller 12 may vary the signalsprovided to actuators 152 and 154 to control the opening and closing ofthe respective intake and exhaust valves. The position of intake valve150 and exhaust valve 156 may be determined by respective valve positionsensors (not shown). The valve actuators may be of the electric valveactuation type or cam actuation type, or a combination thereof. Theintake and exhaust valve timing may be controlled concurrently or any ofa possibility of variable intake cam timing, variable exhaust camtiming, dual independent variable cam timing or fixed cam timing may beused. Each cam actuation system may include one or more cams and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable valve timing (VVT) and/or variable valve lift (VVL)systems that may be operated by controller 12 to vary valve operation.For example, cylinder 14 may alternatively include an intake valvecontrolled via electric valve actuation and an exhaust valve controlledvia cam actuation including CPS and/or VCT. In other embodiments, theintake and exhaust valves may be controlled by a common valve actuatoror actuation system, or a variable valve timing actuator or actuationsystem.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen for example when higher octane fuels or fuelswith higher latent enthalpy of vaporization are used. The compressionratio may also be increased if direct injection is used due to itseffect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including two fuel injectors 166 and 170.Fuel injector 166 is shown coupled directly to cylinder 14 for injectingfuel directly therein in proportion to the pulse width of signal FPW-1received from controller 12 via electronic driver 168. In this manner,fuel injector 166 provides what is known as direct injection (hereafterreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 1shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may aid in mixing and combustion when operating the engine withan alcohol-based fuel due to the lower volatility of some alcohol-basedfuels. Alternatively, the injector may be located overhead and near theintake valve to aid in mixing of intake air and injected fuel. Fuel maybe delivered to fuel injector 166 from fuel system 172 including a fueltank, fuel pumps, a fuel rail, and driver 168. Alternatively, fuel maybe delivered by a single stage fuel pump at lower pressure, in whichcase the timing of the direct fuel injection may be more limited duringthe compression stroke than if a high pressure fuel system is used.Further, although not shown in FIG. 1, the fuel tank may have a pressuretransducer providing a signal to controller 12.

Fuel injector 170 is shown arranged in intake passage 146, rather thanin cylinder 14, in a configuration that provides what is known as portinjection of fuel (hereafter referred to as “PFI”) into the air intakeport upstream of cylinder 14. Fuel injector 170 may inject fuel inproportion to the pulse width of signal FPW-2 received from controller12 via electronic driver 171. Fuel may be delivered to fuel injector 170by fuel system 172.

Fuel may be delivered by both injectors to the cylinder during a singlecycle of the cylinder. For example, each injector may deliver a portionof a total fuel injection that is combusted in cylinder 14. Further, thedistribution and/or relative amount of fuel delivered from each injectormay vary with operating conditions such as described herein below. Therelative distribution of the total injected fuel among injectors 166 and170 may be referred to as a first injection ratio. For example,injecting a larger amount of the fuel for a combustion event via (port)injector 170 may be an example of a higher first ratio of port to directinjection, while injecting a larger amount of the fuel for a combustionevent via (direct) injector 166 may be a lower first ratio of port todirect injection. Note that these are merely examples of differentinjection ratios, and various other injection ratios may be used.Additionally, it should be appreciated that port injected fuel may bedelivered during an open intake valve event, closed intake valve event(e.g., substantially before an intake stroke, such as during an exhauststroke), as well as during both open and closed intake valve operation.Similarly, directly injected fuel may be delivered during an intakestroke, as well as partly during a previous exhaust stroke, during theintake stroke, and partly during the compression stroke, for example.Further, the direct injected fuel may be delivered as a single injectionor multiple injections. These may include multiple injections during thecompression stroke, multiple injections during the intake stroke or acombination of some direct injections during the compression stroke andsome during the intake stroke. When multiple direct injections areperformed, the relative distribution of the total directed injected fuelbetween an intake stroke (direct) injection and a compression stroke(direct) injection may be referred to as a second injection ratio. Forexample, injecting a larger amount of the direct injected fuel for acombustion event during an intake stroke may be an example of a highersecond ratio of intake stroke direct injection, while injecting a largeramount of the fuel for a combustion event during a compression strokemay be an example of a lower second ratio of intake stroke directinjection. Note that these are merely examples of different injectionratios, and various other injection ratios may be used. Furthermore theinjection ratios may be adjusted based on one or more engine operatingconditions such as engine load, engine speed, fuel system pressure,engine temperature, and the like. In this way one or both of liquid andgaseous fuels may be combusted in an engine cylinder.

As such, even for a single combustion event, injected fuel may beinjected at different timings from a port and direct injector.Furthermore, for a single combustion event, multiple injections of thedelivered fuel may be performed per cycle. The multiple injections maybe performed during the compression stroke, intake stroke, or anyappropriate combination thereof.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel injectors 166 and 170 may have different characteristics. Theseinclude differences in size, for example, one injector may have a largerinjection hole than the other. Other differences include, but are notlimited to, different spray angles, different operating temperatures,different targeting, different injection timing, different spraycharacteristics, different locations etc. Moreover, depending on thedistribution ratio of injected fuel among injectors 170 and 166,different effects may be achieved. Further still, fuel injectors 166 and170 may each include one or more gaseous fuel injectors for injectinggaseous fuel, and one or more liquid fuel injectors for injecting liquidfuel.

Fuel system 172 may include one fuel tank or multiple fuel tanks Inembodiments where fuel system 172 includes multiple fuel tanks, the fueltanks may hold fuel with the same fuel qualities or may hold fuel withdifferent fuel qualities, such as different fuel compositions. Thesedifferences may include different alcohol content, different octane,different heat of vaporizations, different fuel blends, and/orcombinations thereof etc. In one example, fuels with different alcoholcontents could include gasoline, ethanol, methanol, or alcohol blendssuch as E85 (which is approximately 85% ethanol and 15% gasoline) or M85(which is approximately 85% methanol and 15% gasoline). Other alcoholcontaining fuels could be a mixture of alcohol and water, a mixture ofalcohol, water and gasoline etc. In some examples, fuel system 172 mayinclude a fuel tank that holds a liquid fuel, such as gasoline, and alsoholds a gaseous fuel, such as CNG. A portion of the gaseous fuel may besolubilized in the liquid fuel. The liquid fuel and the gaseous fueltogether may be referred to as a mixed fuel, and the fuel tank 200 maythus store or hold a mixed fuel. Fuel injectors 166 and 170 may beconfigured to inject fuel from the same fuel tank, from different fueltanks, from a plurality of the same fuel tanks, or from an overlappingset of fuel tanks While FIG. 1 depicts fuel injector 166 as a directfuel injector and fuel injector 170 as a port fuel injector, in otherembodiments both injectors 166 and 170 may be configured as port fuelinjectors or may both be configured as direct fuel injectors.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; and absolute manifold pressure signal (MAP) from sensor124. Engine speed signal, RPM, may be generated by controller 12 fromsignal PIP. Manifold pressure signal MAP from a manifold pressure sensormay be used to provide an indication of vacuum, or pressure, in theintake manifold.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed. Example routines that maybe performed by the controller are described herein and with regards toFIGS. 3 and 4.

Turning now to FIG. 2, it shows a schematic diagram of a multi-cylinderengine in accordance with the present disclosure. As depicted in FIG. 1,internal combustion engine 10 includes cylinders 14 coupled to intakepassage 144 and exhaust passage 148. Intake passage 144 may includethrottle 162. Exhaust passage 148 may include emissions control device178. Control system 13, including controller 12, may receive signalsfrom various sensors 16, and additional sensors shown in FIGS. 1 and 2,and output signals to various actuators 81, including additionalactuators shown in FIGS. 1 and 2.

Cylinders 14 may be configured as part of cylinder head 201. In FIG. 2,cylinder head 201 is shown with 4 cylinders in an inline configuration.In some examples, cylinder head 201 may have more or fewer cylinders,for example six cylinders. In some examples, the cylinders may bearranged in a V configuration or other suitable configuration.

Cylinder head 201 is shown coupled to fuel system 172. Cylinder 14 isshown coupled to fuel injectors 166A and 166B, and fuel injectors 170Aand 170B. Although only one cylinder is shown coupled to fuel injectors,it is to be understood that all cylinders 14 included in cylinder head201 may also be coupled to one or more fuel injectors. In this exampleembodiment, fuel injectors 166A and 166B are depicted as a direct fuelinjector and fuel injectors 170A and 170B are depicted as a port fuelinjector. Although only two direct injectors and two port injectors areshown in FIG. 2, it is to be understood that engine 10 may comprise morethan two direct injectors and more than two fuel injectors. Each fuelinjector may be configured to deliver a specific quantity of gaseousand/or liquid fuel at a specific time point in the engine cycle inresponse to commands from controller 12. One or more fuel injectors maybe utilized to deliver combustible fuel to cylinder 14 during eachcombustion cycle. The timing and quantity of fuel injection may becontrolled as a function of engine operating conditions.

Fuel system 172 includes fuel tank 200. Fuel tank 200 may include aliquid fuel, such as gasoline, diesel fuel, or a gasoline-alcohol blend(e.g. E10, E85, M15, or M85), and may also include a gaseous fuel, suchas CNG. Fuel tank 200 may be configured to store liquid fuel and gaseousfuel together at a relatively low pressure compared to conventional CNGstorage (e.g. 200-250 atmospheres). For example, the gaseous fuel may beadded to a pressure of 100 atmospheres. In this way, a portion of thegaseous fuel may be dissolved in the liquid fuel. At 100 atmospheres,CNG may dissolve in gasoline to the point where 40% of the liquid fuelcomponent in fuel tank 200 is CNG. Fuel tank 200 may include pressuresensor 211, temperature sensor 212, and liquid level sensor 215.

Liquid fuel and/or gaseous fuel may be supplied from fuel tank 200 tocylinders 14 of engine 10 via liquid fuel line 220 and gaseous fuel lineand 221, fuel rails 205 and 206 and fuel injectors 166A, 166B, 170A, and170B. In one example, gaseous fuel may be delivered from fuel tank 200by positioning three-way gaseous fuel switching valve 224 to fluidlycouple fuel tank 200 to gaseous fuel line 221 and gaseous fuel rail 205.Gaseous fuel delivered to gaseous fuel rail 205 may be port fuelinjected to cylinder 14 by gaseous fuel injector 170B, and may bedirectly injected to cylinder 14 by liquid fuel injector 170A. Liquidfuel, including solubilized gaseous fuel in the liquid fuel, may besupplied from fuel tank 200 by operating fuel lift pump 210. Liquid fuelline 220 may be coupled to a lower portion of fuel tank 200 in orderdraw liquid fuel from fuel tank 200 via fuel lift pump 210. In somecases, fuel lift pump 210 may be omitted from fuel system 172. In suchembodiments, the pressure of gaseous fuel stored in fuel tank 200 may beused to drive liquid fuel from fuel tank 200 to fuel rail 205 via fuelline 220. In embodiments where fuel lift pump 210 is omitted, anadditional liquid fuel valve may be coupled to fuel line 220 to controlliquid fuel flow through fuel line 220. If fuel separator bypass valve226 is open, liquid fuel may be delivered via bypass fuel line 228 toliquid fuel line 220 and liquid fuel rail 206, where liquid fuel may bedirectly injected into cylinder 14 via liquid fuel injector 166A and/orport fuel injected into cylinder 14 liquid fuel injector 166B. In thecase of fuel systems comprising more than one fuel tank comprisinggaseous fuel and liquid fuel stored therein, each fuel tank may befluidly coupled to fuel separator 230 to enable separation of thegaseous fuel and the liquid fuel prior to fuel injection.

In one example, gaseous fuel rail 205 may comprise a DI gaseous fuelrail for direct injecting gaseous fuel via one or more DI gaseous fuelinjectors 170A and a PFI gaseous fuel rail for port injection of gaseousfuel via one or more PFI liquid fuel injectors 170B. In other examples,only a DI gaseous or only a PFI gaseous injection system may be used.Furthermore, liquid fuel rail 206 may comprise a DI liquid fuel rail fordirect injecting liquid fuel via one or more DI liquid fuel injectors166A and a PFI liquid fuel rail for port injection of liquid fuel viaone or more PFI liquid fuel injectors 166B. In other examples, only a DIliquid or only a PFI liquid injection system may be used. Further still,a DI gaseous fuel pump may be provided upstream of DI gaseous fuel railand downstream of gaseous fuel switching valve 224 for deliveringpressurized gaseous fuel to DI gaseous fuel rail. Further still, a DIliquid fuel pump may be provided upstream of DI liquid fuel rail anddownstream of bypass fuel line 228 for delivering pressurized liquidfuel to DI liquid fuel rail. Further still, a single DI fuel pump may beused to deliver both gaseous fuel and liquid fuel. Although not shown inFIG. 2, DI liquid fuel pump may be a high pressure fuel pump comprisinga solenoid activated inlet check valve, a piston, and an outlet checkvalve for delivering high pressure liquid fuel to DI liquid fuel rail.Injection of liquid fuel via DI liquid fuel injection pump may lubricatethe piston of liquid DI fuel pump, thereby reducing pump wear anddegradation and reducing pump NVH.

If fuel separator bypass valve is closed, liquid fuel supplied from fueltank 200 may be delivered to fuel separator 230. As an example, fuelseparator 230 may comprise a coalescer or other known processing unitfor separating liquids and gases. Fuel separator 230 may comprise acoalescing filter 234, and an expansion chamber 232 on a downstream sidethereof and a sump chamber 236 on an upstream side thereof. Liquid fuelmay be supplied to the sump chamber 236 and/or the coalescing filter 234at a fuel pressure of fuel lift pump 210. A pressure differential may bemaintained across coalescing filter 234 wherein a pressure in expansionchamber 232 may be less than the fuel pressure in the sump chamber 236.For example, the pressure differential may be maintained by controllinglift pump 210 to supply sufficient pressure. Furthermore, coalescingfilter may comprise a fritted filter, such as a steel fitted filter.Expansion chamber 232 may also be described as a manifold chamber.

In one example, the pressure differential may be maintained acrosscoalescing filter 234 by a check valve 242 positioned downstream of thefuel separator 230 and fluidly coupled to the expansion chamber 232 viaa gaseous fuel relief passage 240. The check valve 242 may be configuredto open when a pressure upstream of the check valve exceeds a thresholdpressure, for example, an intake manifold pressure. The outlet of thecheck valve 242, via the gaseous fuel relief passage 240, may be fluidlycoupled to the intake manifold and/or a positive crankcase ventilation(PCV) system of engine 10. In this way, separated gaseous fuel may besupplied to the intake manifold and/or the engine crankcase where it maybe used to aid in reducing oil viscosity and in lubricating the enginecomponents.

For example, a pressure in the expansion chamber 232 may be less than athreshold pressure. The pressure in expansion chamber 232 may bemeasured by pressure sensor 238 and communicated to controller 12. Inone example, the threshold pressure may comprise a pressure less than100 psi. Below 100 psi, the solubility of CNG, methane, and othergaseous fuels may be reduced such that an amount of solubilized gaseousfuel in the liquid fuel may be very low. For example, the solubility ofgaseous fuel (volume gaseous fuel dissolved per volume of liquid fuel)may be approximately 1 mL/mL per atmospheres of gaseous fuel pressure.Accordingly, upon entering the fuel separator 230, gaseous fuelsolubilized in the liquid fuel may be desolubilized and volatilized fromthe liquid fuel, and may be conveyed across the coalescing filter 234into the expansion chamber 232 and out of fuel separator 230 towardsgaseous fuel switching valve 224. Furthermore, positioning the three-waygaseous fuel switching valve 224 to fluidly connect the fuel separator230 to gaseous fuel rail 205 may supply the desolubilized gaseous fuelto gaseous fuel rail 205. Subsequently, desolubilized gaseous fuel maybe injected to cylinder 14 via gaseous fuel injectors 170A and 170B.

Volatilization of the gaseous fuel from the liquid fuel may decrease theliquid and gaseous fuel temperatures and may cool the coalescing filter234. Furthermore, the lowered liquid fuel temperature may reduce thevolatility of the liquid fuel so that entrainment of lighter hydrocarbonfuel components into the desolubilized gaseous fuel stream may bereduced. Any entrained hydrocarbon components in the liquid fuel, suchas residual butanes, pentanes, and hexanes, may volatilize and beentrained by the desolubilized gaseous fuel in the fuel separator.Accordingly, the octane value of the liquid fuel may thus be slightlyraised, while the octane value of the gaseous fuel may be slightlyreduced. Furthermore, recompression of the gaseous fuel downstream ofthe fuel separator may condense these residual hydrocarbon components.

Liquid fuel delivered to the fuel separator 230 may flow through sumpchamber 236 and out of fuel separator 230 towards bypass fuel line 228and liquid fuel line 220. A portion of liquid fuel delivered to fuelseparator 230 may condense as droplets on and within coalescing filter234. As the droplets of liquid fuel flow through the coalescing filter234, they may merge and coalesce, thereby forming larger droplets, whichmay be conveyed by gravity back to the sump chamber 236.

Although in FIG. 2, the sump chamber 236, coalescing filter 234, andexpansion chamber 232 are illustrated as being arranged in a linearfashion, other arrangements may also be included. For example, the sumpchamber 236, coalescing filter 234, and expansion chamber 232 may bearranged in a concentric configuration wherein the expansion chamber isencircled by the coalescing filter 234 and the sump chamber 236, andwherein the fuel flowing into and out of the fuel separator 230 flows inan axial and/or radial direction with respect to the concentricconfiguration. Furthermore, sump chamber 236 may be fluidly connected toexpansion chamber 232 via coalescing filter 234.

In this way, solubilized gaseous fuel in the liquid fuel may bedesolubilized and separated from the liquid fuel before injection ofliquid fuel into cylinder 14. Furthermore, gaseous fuel may be injectedseparately from liquid fuel to cylinder 14 via gaseous fuel injectors170A and 170B. In other words, gaseous fuel may be injected only viagaseous fuel injectors and liquid fuel may be injected only via liquidfuel injectors. Furthermore, only gaseous fuel may be injected byswitching off liquid fuel injection, or only liquid fuel may be injectedby switching off gaseous fuel injection. Gaseous fuel may comprise oneor more of compressed natural gas (CNG), methane, propane, and butane asnon-limiting examples, while liquid fuel may comprise one or more ofgasoline, alcohol, and diesel fuel, as non-limiting examples.

For example, injection of gaseous fuel may be increased because gaseousfuel may be lower cost, lower carbon intensity (e.g., lower CO₂generating), higher octane, and the like, relative to liquid fuel.However, at high engine loads (especially when port fuel injectinggaseous fuel), injection of only gaseous fuel without injection ofliquid fuel may reduce engine operability because the gaseous fuel maydisplace air (e.g., intake air entering the cylinder and/or at theintake air passage) and reduce engine efficiency (e.g., adversely alterthe fuel to air ratio). Thus, at engine loads greater than a thresholdload, injection of solubilized gaseous fuel in liquid fuel may beperformed. Furthermore, at engine loads greater than a threshold loadand when port fuel injection is ON, injection of solubilized gaseousfuel in liquid fuel may be performed.

Separation of solubilized gaseous fuel from the liquid fuel may beperformed when engine operating conditions and fuel system conditionsmay lead to fuel delivery issues arising from generation of gaseous fuelbubbles in the liquid fuel. For example, under high fuel systemtemperatures (e.g., high ambient temperature or high under-hoodtemperature), under low fuel system pressure, gaseous fuel may be likelyto form bubbles as the solubilized gaseous fuel in liquid fuel isconveyed through the fuel system towards the cylinder. Thus, underconditions where an ambient temperature is greater than a thresholdambient temperature, an under-hood temperature is greater than athreshold under-hood temperature, or a fuel system pressure is lowerthan a threshold fuel system pressure, fuel separator bypass valve 226may be closed to direct liquid fuel through fuel separator 230, therebyseparating solubilized gaseous fuel from liquid fuel in order to reduceformation of gaseous fuel bubbles and to increase fuel deliveryreliability and robustness. Ambient temperature and under-hoodtemperature may be measured by one or more temperature sensors installedunder a vehicle hood (e.g., near the fuel lines 228 220, or near thefuel separator 230, or temperature sensor 212 at fuel tank 200), mountedon a front vehicle bumper, or behind the vehicle front grill. Fuelsystem pressure may be measured by one or more pressure sensorspositioned at fuel system 172. For example fuel system pressure may bemeasured by one or more of pressure sensor 211 at fuel tank 200,pressure sensor 238 at fuel separator 230, pressure sensor 223 in liquidfuel line 220, and a pressure sensor in either of gaseous fuel rail 205or liquid fuel rail 206.

Solubilized gaseous fuel may also be separated from liquid fuel duringconditions when the engine temperature is less than a threshold enginetemperature because particulate emissions can increase when fueldroplets strike colder metal surfaces in the engine and fail toevaporate. Furthermore, amounts of liquid fuel on metal surfaces mayalso be increased at certain engine speed and engine load conditionswhere air fluid dynamics in the engine system entrain and direct thefuel spray closer to the walls of the engine system, such as thecylinder walls. Therefore controlling the fuel injector spray patternmay aid in controlling generation of particulate emissions duringcertain operating conditions when particulate emissions are likely. Inone example, a look-up table may be used by control system 13 topredetermine engine load and engine speed conditions when particulategeneration may occur. As such, under these predetermined engine load andengine speed conditions, solubilized gaseous fuel may be separated fromliquid fuel so that liquid fuel injection may be reduced (and gaseousfuel injection may be increased) to reduce particulate emissions.

Fuel system 172 is shown coupled to refueling system 250. Refuelingsystem 250 may be coupled to fuel tank 200 via tank access valve 218.Tank access valve 218 may be coupled to refueling conduit 260. Refuelingconduit 260 may include high pressure refueling port 255. High pressurerefueling port 255 may be configured to receive a pressurized gaseousfuel pump nozzle, or a fuel pump nozzle configured to deliver apre-pressured mixture of liquid fuel and gaseous fuel. In some cases, asecond high pressure refueling port may be included to allowcompatibility with more than one type of high pressure fuel pump nozzle.

Access to high pressure refueling port 255 may be regulated by refuelinglock 257. In some embodiments, refueling lock 257 may be a fuel caplocking mechanism. The fuel cap locking mechanism may be configured toautomatically lock a fuel cap in a closed position so that the fuel capcannot be opened. For example, the fuel cap may remain locked viarefueling lock 257 while pressure in the fuel tank is greater than athreshold. A fuel cap locking mechanism may be a latch or clutch, which,when engaged, prevents the removal of the fuel cap. The latch or clutchmay be electrically locked, for example, by a solenoid, or may bemechanically locked, for example, by a pressure diaphragm.

In some embodiments, refueling lock 257 may be a filler pipe valvelocated at a mouth of refueling conduit 260. In such embodiments,refueling lock 257 may prevent the insertion of a refueling pump intorefueling conduit 260. The filler pipe valve may be electrically locked,for example by a solenoid, or mechanically locked, for example by apressure diaphragm.

In some embodiments, refueling lock 257 may be a refueling door lock,such as a latch or a clutch which locks a refueling door located in abody panel of the vehicle. The refueling door lock may be electricallylocked, for example by a solenoid, or mechanically locked, for exampleby a pressure diaphragm.

In embodiments where refueling lock 257 is locked using an electricalmechanism, refueling lock 257 may be unlocked by commands fromcontroller 12. In embodiments where refueling lock 257 is locked using amechanical mechanism, refueling lock 257 may be unlocked via a pressuregradient.

Refueling conduit 260 may be coupled to low pressure refueling conduit280. Low pressure refueling conduit 280 may be coupled to surge tank270. Surge tank 270 may include a low pressure refueling port 265 and aliquid sensor 275. Low pressure refueling conduit 280 may include fuelpump 285 and check valve 290. Fuel pump 285 may only operate when fueltank pressure is below a threshold, and may only operate when there isliquid fuel in surge tank 270, as sensed by liquid sensor 275. In thisway, fuel pump 285 may not pump an air/fuel mixture into fuel tank 200.Further, when fuel tank pressure reaches a threshold, fuel pump 285 maybe shut off by controller 12, causing liquid fuel to accumulate in surgetank 270. This may cause a low pressure liquid fuel dispenser nozzleengaged with low pressure refueling port 265 to turn itself off. Accessto refueling port 265 may be regulated by refueling lock 267. Refuelinglock 267 may comprise one of the examples described for refueling lock257. Refueling locks 257 and 267 may further comprise differentmechanisms.

In this manner, a fuel system may comprise a fuel tank on board avehicle, the fuel tank including a gaseous fuel dissolved in a liquidfuel, a fuel injection system downstream from the fuel tank, the fuelinjection system including gaseous fuel injectors and liquid fuelinjectors, and a fuel separator fluidly coupled between the fuel tankand the fuel injection system. The fuel system may further comprise abypass valve fluidly coupled between the fuel tank and the fuelinjection system, wherein upon opening the bypass valve, fuel from thefuel tank is supplied to the fuel injection system while bypassing thefuel separator. The fuel separator may comprise a coalescing filter andan expansion chamber downstream from the coalescing filter, wherein apressure in the expansion chamber is less than a pressure upstream ofthe coalescing filter, and upon filtering fuel through the coalescingfilter, the gaseous fuel is desolubilized into the expansion chamber,and the liquid fuel is coalesced and retained upstream of the coalescingfilter. The coalescing filter may comprise a steel fitted filter.Furthermore, the fuel system may further comprise a controller,including instructions executable to during a first condition,desolubilize and separate the gaseous fuel from the liquid fuel in thefuel separator, and supply the gaseous fuel and the liquid fuel to fuelinjectors, and during a second condition, supply fuel from a fuel tankto the fuel injectors while bypassing the fuel separator.

Turning now to FIG. 3, it illustrates an example flow chart for a method300 of operating an engine system and a fuel system. Method 300 may beexecuted by control strategy of controller 12 of control system 13.Method 300 begins at 310 where engine operating conditions such asengine on condition (EOC), engine temperature, fuel system pressure,engine torque, engine load, engine speed (RPM) and the like are measuredand/or estimated. Method 300 continues at 320 where it determines ifengine operating conditions may increase generation of particulateemissions. At 324, method 300 determines if an engine temperature,T_(engine), is less than a threshold engine temperature, T_(engine,TH).If T_(engine)<T_(engine,TH), liquid fuel droplets may fail to evaporatewhen striking metal surfaces of the engine and may thereby increasegeneration of particulate emissions. If T_(engine) is not less thanT_(engine,TH), method 300 continues at 328 where particulate generationis determined based on an engine speed and load. As an example, method300 may reference a look-up table of predetermined engine speed and loadto determine if particulate emissions may increase at the current enginespeed and load conditions. If method 300 determines that particulateemissions may not increase or are low at the current engine speed andload, method 300 continues at 330.

At 330, method 300 determines if a gaseous fuel bubble formation mayoccur in the fuel system. At 334, method 300 determines if an ambienttemperature, T_(ambient), is greater than a threshold ambienttemperature, T_(ambient,TH). T_(ambient) may also refer to a measured orestimated under-hood temperature or a fuel system temperature, asdescribed above. If T_(ambient)>T_(ambient,TH) gaseous fuel bubbles maybe generated during fuel delivery to the engine, and fuel deliveryreliability and robustness may be reduced. In one example, a fueldelivery volumetric flow rate may be lowered because of the expansion ofgaseous fuel bubbles in a liquid fuel line. In another example,formation of gaseous fuel bubbles may cause cavitation in a liquid fuelline or at a DI fuel pump, reducing fuel delivery reliability anddecreasing engine operability. In yet another example, fuel bubbles mayaffect fuel metering with fuel injectors, thus changing air/fuel ratioand degrading engine emissions. If T_(ambient) is not greater thanT_(ambient,TH), method 300 continues at 338 where it determines if afuel system pressure, P_(fuelsys), is less than a threshold fuel systempressure, P_(fuelsys,TH). If P_(fuelsys)<P_(fuelsys,TH), gaseous fuelbubbles may be generated during fuel delivery to the engine, and fueldelivery reliability and robustness may be reduced. As described above,P_(fuelsys) may be determined from one or more pressure sensorspositioned at fuel system 172. For example fuel system pressure may bemeasured by one or more of pressure sensor 211 at fuel tank 200,pressure sensor 238 at fuel separator 230, pressure sensor 223 in liquidfuel line 220, and a pressure sensor in either of gaseous fuel rail 205or liquid fuel rail 206.

If at 338, P_(fuelsys)<P_(fuelsys,TH), method 300 continues at 340 whereit determines if an engine load is less than a threshold engine load,Load_(TH). Injection of gaseous fuel (especially via port fuelinjection) may displace intake air in the engine cylinder or the engineintake air passage 146. As such, at high engine loads above Load_(TH),displacing engine intake air may reduce available engine torque anddecrease drivability and solubilized gaseous fuel may not be separatedfrom liquid fuel to enable injection of solubilized gaseous fuel inliquid fuel. If engine load is not less than Load_(TH), method 300continues at 360, where it opens the fuel separator bypass valve 226thereby directing the solubilized gaseous fuel and liquid fuel to bypassfuel line 228 and liquid fuel line 220. Furthermore, method 300 mayposition gaseous fuel switching valve 224 to fluidly connect fuel tank200 with gaseous fuel line 221. Next, at 364, the mixture of solubilizedgaseous fuel and liquid fuel may be injected via liquid fuel rail 206and liquid fuel injectors 166A and 166B to the engine. Because method300 determines that engine operating conditions are not conducive togeneration of particulate emissions at 320, and are not conducive togeneration of gaseous fuel bubbles at 330, the mixture of solubilizedgaseous fuel and liquid fuel may be injected to the engine whilemaintaining fuel delivery reliability and robustness. After 364, method300 ends.

If at 324 T_(engine)<T_(engine,TH), at 328 particulate generation mayincrease, at 334 T_(ambient)>T_(ambient,TH), at 338P_(fuelsys)<P_(fuelsys,TH), or at 340 engine load<Load_(TH), method 300proceeds to desolubilize and separate the gaseous fuel from the liquidfuel. At 370, method 300 closes the fuel separator bypass valve andpositions gaseous fuel switching valve 224 to connect fuel separator 230and gaseous fuel line 221. Next, at 372, method 300 directs thesolubilized gaseous fuel and the liquid fuel to the fuel separator 230,for example, via fuel lift pump 210. Alternately, the pressure in fueltank 210 may convey fuel from fuel tank 210. At 374, the gaseous fuel isdesolubilized and separated from the liquid fuel in fuel separator 230.As an example, a pressure differential across coalescing filter 234 maydesolubilize the gaseous fuel, whereby the gaseous fuel flows across thecoalescing filter 234 to expansion chamber 232, and out of fuelseparator 230 through gaseous fuel switching valve 224 to gaseous fuelline 221. As an example, a pressure in the expansion chamber may be lessthan a threshold pressure, for example less than 100 psi, in order toreadily desolubilize the gaseous fuel from the liquid fuel. Liquid fueldroplets may condense on coalescing filter 234 where they may coalesceand then drop back to the sump chamber 236 where most of the liquid fuelcollects before flowing out of fuel separator 230 to liquid fuel bypassline 228 and liquid fuel line 220. After the gaseous fuel and liquidfuel are separated in fuel separator 230, the gaseous fuel and theliquid fuel may be separately injected to the engine via the gaseousfuel injection system and the liquid fuel injection system at 376 and378, respectively. Method 300 ends following 376 and 378.

In this manner, a method for an engine may comprise, on board a vehicle,supplying fuel from a fuel tank to a fuel separator, wherein the fuelcomprises a gaseous fuel solubilized in a liquid fuel, desolubilizingthe gaseous fuel from the liquid fuel in the fuel separator, andseparating the gaseous fuel from the liquid fuel in the fuel separator.The method may further comprise injecting the desolubilized gaseous fuelinto the engine via a gaseous fuel injector, and injecting the liquidfuel into the engine via a liquid fuel injector. Furthermore,desolubilizing the gaseous fuel from the liquid fuel may comprisefiltering the fuel via a coalescing filter in the fuel separator.Further still, desolubilizing the gaseous fuel from the liquid fuel maycomprise reducing a fuel pressure below a threshold pressure. Furtherstill, the threshold pressure may comprise 100 psi. Further still, thegaseous fuel may be separated from the liquid fuel upstream of a fuelinjector. The method may further comprise desolubilizing and separatingthe gaseous fuel from the liquid fuel in the fuel separator when anengine load is less than a threshold load.

In this manner, a method of delivering fuel to an engine may comprise,on board a vehicle, storing fuel in a fuel tank, wherein the fuelcomprises a gaseous fuel solubilized in a liquid fuel, during a firstcondition, desolubilizing and separating the gaseous fuel from theliquid fuel in a fuel separator, and supplying the gaseous fuel and theliquid fuel to fuel injectors, and during a second condition, supplyingfuel from the fuel tank to the fuel injectors while bypassing the fuelseparator. The first condition may comprise when an engine load is lessthan a threshold engine load. Furthermore, the first condition maycomprise when an engine temperature is less than a threshold enginetemperature. Further still, the first condition may comprise when a fuelsystem pressure is lower than a threshold pressure. The second conditionmay comprise when an engine load is greater than a threshold engineload, when an engine temperature is greater than a threshold enginetemperature, and when a fuel system pressure is lower than a thresholdfuel system pressure. Furthermore, the fuel injectors may comprisegaseous fuel injectors and liquid fuel injectors, and the method mayfurther comprise supplying the gaseous fuel to the gaseous fuelinjectors and supplying the liquid fuel and the fuel to the liquid fuelinjectors. Desolubilizing the gaseous fuel may comprise reducing a fuelpressure in the fuel separator below 100 psi. Desolubilizing the gaseousfuel may further comprise filtering the fuel through a coalescingfilter.

Turning now to FIG. 4, it illustrates an example timeline 400 foroperating an engine system and a fuel system, comprising a gaseous fueland a liquid fuel. Timeline 400 includes timelines for fuel separatorbypass valve status 910, gaseous fuel switching valve status 920, engineload 930, engine temperature 940, ambient temperature 950, fuel systempressure 960, and gas injection system status 970. Furthermore, timeline400 includes Load_(TH) 934, T_(engine,TH) 944, T_(ambient,TH) 954, andP_(fuelsys,TH) 964. Prior to time t1, an engine load is less thanLoad_(TH), T_(engine)>T_(engine,TH), T_(ambient)<T_(ambient,TH),P_(fuelsys)>P_(fuelsys,TH), and gas injection system status is ON.Because particulate emission generation and gaseous fuel bubblegeneration resulting from engine operating conditions are low, a fuelseparator bypass valve is OPEN and a gaseous fuel switching valve ispositioned to fluidly couple the fuel tank with the gaseous fuel line.In this way a mixture of solubilized gaseous fuel and liquid fuel may beinjected via liquid fuel injection system to engine.

At time t1, the engine load increases above Load_(TH), for example whenascending a steep hill. In response, the fuel separator bypass valve mayremain open and the gaseous fuel switching valve may remain positionedto couple a gaseous fuel line to the fuel tank to enable injection of amixture of solubilized gaseous fuel and liquid fuel. Furthermore,gaseous fuel injection system status may be switched to OFF sinceinjection of gaseous fuel at high loads may displace intake air,reducing available engine torque and reducing engine operability. Inother examples, in response to an engine load increasing aboveLoad_(TH), gaseous fuel injection may be decreased. At time t2, engineload decreases below Load_(TH). In response, the gaseous fuel injectionsystem status is switched ON. Injection of gaseous fuel may be lowercost, lower carbon intensity (e.g., lower CO₂ generating), higheroctane, and the like, relative to liquid.

At time t3, T_(engine) decreases below T_(engine,TH). In response, afuel separator bypass valve may be closed and a gaseous fuel switchingvalve may be positioned to fluidly connect a fuel separator with agaseous fuel line. In this way, solubilized gaseous fuel may beseparated from liquid fuel, and gaseous fuel may be separately injectedfrom liquid fuel. Since T_(engine)<T_(engine,TH), particulate emissionsmay be increased when injected liquid fuel contacts cold metal enginesurfaces. Separation of the gaseous fuel from the liquid fuel may enableincreasing gaseous fuel injection while decreasing liquid fuel injectionto reduce generation of particulate emissions. At time t4, T_(engine)increases above T_(engine,TH). In response, fuel separator bypass valveis opened and gaseous fuel switching valve is closed to allow injectionof a mixture of solubilized gaseous fuel and liquid fuel.

At time t5, T_(ambient) increases above T_(ambient,TH). In response, afuel separator bypass valve may be closed and a gaseous fuel switchingvalve may be positioned to fluidly connect a fuel separator with agaseous fuel line. In this way, solubilized gaseous fuel may beseparated from liquid fuel, and gaseous fuel may be separately injectedfrom liquid fuel. Since T_(ambient)>T_(ambient,TH) generation of gaseousfuel bubbles may increase when solubilized gaseous fuel in liquid fuelis conveyed in the fuel system and injected to the engine. Separation ofthe gaseous fuel from the liquid fuel may enable reducing generation ofgaseous fuel bubbles in the fuel system, thereby increasing reliabilityand robustness of fuel delivery while separately injecting gaseous fueland liquid fuel and while maintaining engine operability and vehicledrivability. At time t6, T_(ambient) decreases below T_(ambient,TH). Inresponse, fuel separator bypass valve is opened and gaseous fuelswitching valve is closed to allow injection of a mixture of solubilizedgaseous fuel and liquid fuel.

At time t7, P_(fuelsys) decreases below P_(fuelsys,TH). In response, afuel separator bypass valve may be closed and a gaseous fuel switchingvalve may be positioned to fluidly connect a fuel separator with agaseous fuel line. In this way, solubilized gaseous fuel may beseparated from liquid fuel, and gaseous fuel may be separately injectedfrom liquid fuel. Since P_(fuelsys)<P_(fuelsys,TH), generation ofgaseous fuel bubbles may increase when solubilized gaseous fuel inliquid fuel is conveyed in the fuel system and injected to the engine.Separation of the gaseous fuel from the liquid fuel may enable reducinggeneration of gaseous fuel bubbles in the fuel system, therebyincreasing reliability and robustness of fuel delivery while separatelyinjecting gaseous fuel and liquid fuel and while maintaining engineoperability and vehicle drivability. At time t8, P_(fuelsys) increasesabove P_(fuelsys,TH). In response, fuel separator bypass valve is openedand gaseous fuel switching valve is closed to allow injection of amixture of solubilized gaseous fuel and liquid fuel.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations,and/or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedactions, operations and/or functions may be repeatedly performeddepending on the particular strategy being used. Further, the describedactions, operations and/or functions may graphically represent code tobe programmed into non-transitory memory of the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method for an engine, comprising: on board a vehicle, supplyingfuel from a fuel tank to a fuel separator, wherein the fuel comprises agaseous fuel solubilized in a liquid fuel; desolubilizing the gaseousfuel from the liquid fuel in the fuel separator; and separating thegaseous fuel from the liquid fuel in the fuel separator.
 2. The methodof claim 1, further comprising injecting the desolubilized gaseous fuelinto the engine via a gaseous fuel injector, and injecting the liquidfuel into the engine via a liquid fuel injector.
 3. The method of claim1, wherein desolubilizing the gaseous fuel from the liquid fuelcomprises filtering the fuel via a coalescing filter in the fuelseparator.
 4. The method of claim 1, wherein desolubilizing the gaseousfuel from the liquid fuel comprises reducing a fuel pressure below athreshold pressure.
 5. The method of claim 3, wherein the pressure inthe filter's expansion chamber is less than 100 psi.
 6. The method ofclaim 1, wherein the gaseous fuel is separated from the liquid fuelupstream of a fuel injector.
 7. The method of claim 1, furthercomprising desolubilizing and separating the gaseous fuel from theliquid fuel in the fuel separator when an engine load is less than athreshold load.
 8. A method of delivering fuel to an engine, comprising:on board a vehicle, storing fuel in a fuel tank, wherein the fuelcomprises a gaseous fuel solubilized in a liquid fuel; during a firstcondition, desolubilizing and separating the gaseous fuel from theliquid fuel in a fuel separator, and supplying the gaseous fuel and theliquid fuel to fuel injectors; and during a second condition, supplyingmixed fuel from the fuel tank to the fuel injectors while bypassing thefuel separator.
 9. The method of claim 8, wherein the first conditioncomprises when an engine load is less than a threshold engine load. 10.The method of claim 8, wherein the first condition comprises when anengine temperature is less than a threshold engine temperature.
 11. Themethod of claim 8, wherein the first condition comprises when a fuelsystem pressure is lower than a threshold pressure or when an ambienttemperature is lower than a threshold temperature.
 12. The method ofclaim 8, wherein the second condition comprises when an engine load isgreater than a threshold engine load, when an engine temperature isgreater than a threshold engine temperature, when a fuel system pressureis greater than a threshold fuel system pressure, or when an ambienttemperature is higher than a threshold temperature.
 13. The method ofclaim 8, wherein the fuel injectors comprise gaseous fuel injectors andliquid fuel injectors, and further comprising supplying the gaseous fuelto the gaseous fuel injectors and supplying the liquid fuel and themixed fuel to the liquid fuel injectors.
 14. The method of claim 8,wherein desolubilizing the gaseous fuel comprises reducing a fuelpressure in the fuel separator below 100 psi.
 15. The method of claim14, wherein desolubilizing the gaseous fuel further comprises filteringthe fuel through a coalescing filter.
 16. A fuel system, comprising: afuel tank on board a vehicle, the fuel tank including a gaseous fueldissolved in a liquid fuel; a fuel injection system downstream from thefuel tank, the fuel injection system including gaseous fuel injectorsand liquid fuel injectors; and a fuel separator fluidly coupled betweenthe fuel tank and the fuel injection system.
 17. The fuel system ofclaim 16, further comprising a bypass valve fluidly coupled between thefuel tank and the fuel injection system, wherein upon opening the bypassvalve, fuel from the fuel tank is supplied to the fuel injection systemwhile bypassing the fuel separator.
 18. The fuel system of claim 17,wherein the fuel separator comprises a coalescing filter and anexpansion chamber downstream from the coalescing filter, wherein apressure in the expansion chamber is less than a pressure upstream ofthe coalescing filter, and upon filtering fuel through the coalescingfilter, the gaseous fuel is desolubilized into the expansion chamber,and the liquid fuel is coalesced and retained upstream of the coalescingfilter.
 19. The fuel system of claim 18, wherein the coalescing filtercomprises a fritted filter.
 20. The fuel system of claim 16, furthercomprising a controller, including instructions executable to: during afirst condition, desolubilize and separate the gaseous fuel from theliquid fuel in the fuel separator, and supply the gaseous fuel and theliquid fuel to fuel injectors, and during a second condition, supplymixed fuel from a fuel tank to the fuel injectors while bypassing thefuel separator.